I. Technical Field
The present invention is related to a fan which is disposed within a casing of an electronic device and discharges air within the casing to an outside of the casing, and also related to an electronic device in which the fan is mounted.
II. Description of the Related Art
Among various electronic devices, in those devices in which the amount of heat release within the device is large, a fan which discharges air within the device to the outside is generally mounted for cooling the inside. The majority of such fans are disposed near an inner surface of an outer wall of the electronic device in parallel with the inner surface. In recent years, many electronic devices equipped with a fan have been miniaturized or are thinner. Therefore, the space in which the fan is disposed has also become smaller. In other words, a gap between a portion of the fan on the suction side and parts, circuit boards, chassis, and the like mounted within the electronic device has become smaller.
However, as this gap becomes smaller, issues of an increase in noise and a reduction in air flow occur with conventional fans. In a conventional fan, if the portion of the fan on the suction side gets too close to the parts, circuit boards, chassis, and the like, an issue occurs in that an exhaust flow amount rapidly decreases.
The constitutions of a conventional fan and an electronic device equipped with such a fan will be explained below using FIGS. 24A to 26C.
FIG. 24A is a perspective view from the suction side of a conventional fan, and FIG. 24B is a perspective view from the exhaust side of a conventional fan. FIG. 25A is a plan view from the suction side of a conventional fan, and FIG. 25B is a plan view from the exhaust side of a conventional fan. FIG. 25C is a partial cross-section view taken along line Aa-Aa of the fan shown in FIG. 25A, and FIG. 25D is a partial cross-section view taken along line B-B of the fan shown in FIG. 25A. FIG. 26A is a plan view from the rear surface side of an electronic device equipped with a conventional fan. FIG. 26B is a partial cross-section view taken along line Ab-Ab of FIG. 26A, and FIG. 26C is a partially enlarged cross-section view of FIG. 26B. The outline of a blade 105 shown in FIGS. 25C, 25D, and 26C shows a rotation trajectory when the blade 105, is rotating.
First, the constitution of a conventional fan will be explained below.
In FIGS. 29A to 25D, a conventional fan 101 is generally called an axial flow fan, and the fan 101 mainly includes impeller 102 and a substantially rectangular casing 103. The impeller 102 includes a plurality of propeller-shaped blades 105, and a cylindrical hub 109 to which the blades 105 are attached. A motor 110 which rotates centered on a rotation axis 106 of the impeller 102 is mounted within the hub 109.
The motor 110 is supported by a motor-base 112. The motor-base 112 is connected and fixed to the casing 103 by four connecting strips 119a to 119d. 
The casing 103 surrounds a periphery of the blades 105 and includes an inner wall which acts as an air duct. Tapered portions 107a, 107b, which are inclined such that their distance from the rotation axis 106 gradually expands towards the suction side, are formed on the inner wall of the casing 103 as shown in FIGS. 25C and 25D. As shown in FIG. 25A, the tapered portions 107a and the tapered portions 107b are each alternately provided at four locations on the inner wall. The tapered portions 107a are formed such that their inclination angle relative to the rotation axis 106 is larger than that of the tapered portions 107b. Tapered portions 106a, 106b, whose distance from the rotation axis 106 gradually expands towards the exhaust side, are formed on the inner wall of the casing 103 as shown in FIGS. 25C and 25D. As shown in FIG. 25B, the tapered portions 106a and the tapered portions 106b are each alternately provided at four locations on the inner wall. The tapered portions 106a are formed such that their inclination angle relative to the rotation axis 106 is larger than that of the tapered portions 106b. Cylindrical straight portions 108 are provided between the tapered portions 107a and the tapered portions 106a, and between the tapered portions 107b and the tapered portions 106b. Each of the straight portions 108 is disposed such that there is a slight gap between the straight portions 108 and an outer edge 109 that forms an outer periphery of the rotation trajectory of the blades 105. Mounting holes 150a to 150d for mounting other members are provided at the four corners of the fan 101.
Next, the constitution of an electronic device equipped with the conventional fan will be explained.
As shown in FIGS. 26A to 26C, an electronic device 124 includes a chassis 126, a circuit board 127, and a fan 101. A rear surface cover 130 engages with the chassis 126 and is fixed with a screw (not illustrated). The circuit board 127 is fixed to a boss (not illustrated) mounted upright on the chassis 126. An electronic component (not illustrated) is mounted on a mounting region 131 upon the circuit board 127. Four bosses (not illustrated) mounted upright on the chassis 126 are inserted into mounting holes 150a to 150d of the fan 101, and the portion of the fan 101 on the suction side is screw fixed so that it faces the chassis 126 in parallel.
A sponge 134 is attached in a gap between an outer periphery on the exhaust side of the casing 103 of the fan 101 and the rear surface cover 130 so that discharged air does not return to the inside of the electronic device 124 from the gap. A plurality of suction holes 132 and an exhaust holes 133 are provided in the shape of small circular holes on the rear surface cover 130.
In the electronic device 124 constituted as described above, the electronic component on the mounting region 131 of the circuit board 127 becomes a heat source and releases heat. This heat is transmitted to the air within the electronic device 124 from a front surface of the electronic component and a front surface of the circuit board 127.
Next, the air flow by the fan 101 mounted in the electronic component 124 will be explained. As shown in FIG. 26C, a distance in a height direction between an inner surface of the chassis 126 and the edge of the casing 103 on the suction side is defined as a suction distance h3, and a distance in a height direction from the inner surface of the chassis 126 to an inner surface of the rear surface cover 130 is defined as inner height h4.
First, the air flow in the case that the suction distance h3 is sufficiently large will be explained.
When the fan 101 rotates in the direction of arrow 111 shown in FIG. 24A, air is sucked in from the suction holes 132 shown in FIGS. 26A and 26B. The sucked air passes between the electronic component on the circuit board 127 or through a gap between the circuit board 127 and the chassis 126, and then flows to the vicinity of the portion of the casing 103 on the suction side. Afterwards, the air passes between the inner wall of the casing 103 and the hub 104, and is discharged from the exhaust holes 133.
However, in the electronic device 124 including the conventional fan 101, an exhaust flow amount of the fan 101 decreases as the suction distance h3 gets smaller, as shown by a dashed line in the graph of FIG. 27. In order to eliminate any influence from the circuit board 127, the dashed line in the graph of FIG. 27 shows an exhaust flow amount when the suction distance h3 shown in FIG. 26C is changed using a tester wherein the circuit board 127 was removed from the electronic device 124 shown in FIG. 26A.
In the conventional fan 101, as shown by the dashed line in FIG. 27, as the suction distance h3 becomes smaller, the exhaust flow amount decreases. When the suction distance h3 is 6 mm, the exhaust flow amount becomes zero. This phenomenon occurs because air flows in reverse from the exhaust holes 133 of the rear surface cover 130 and spiral flows 136 in which air flows to the outside again increase, as shown by arrows in FIG. 26C. Therefore, in the conventional fan 101, an issue arises in that obstructions such as the chassis 126, parts, and boards cannot be put greatly close to the portion on the suction side due to the reduction in the exhaust flow amount.
As another conventional example, a technology for improving the air flow of the fan is disclosed in, for example, Japanese Unexamined Utility Model Application No. H06-004399. Japanese Unexamined Utility Model Application No. H06-004399 discloses a fan which aims to increase the air flow by further enlarging the inclination angle of the tapered portions on the exhaust side of the fan toward the exhaust side.
As a further conventional example, a technology for improving the thinness of a fan is disclosed in, for example, Japanese Unexamined Patent Application No. H05-044697, Japanese Unexamined Patent Application No. H05-044697 discloses a circular cone-shaped mixed-flow fan in which a radius of an outer peripheral surface of a hub to which propeller-shaped blades of an impeller are attached increases towards the exhaust side.
As another conventional example, Japanese Unexamined Patent Application No. 2003-269393, for example, discloses using a centrifugal-type multiblade fan, and also discloses a fan using a centrifugal-type blade shape in which a suction inlet and an exhaust outlet are disposed in a front-rear relationship.    Patent Document 1: Japanese Unexamined Utility Model Application No. H06-004399    Patent Document 2: Japanese Unexamined Patent Application No. H05-044697    Patent Document 3: Japanese Unexamined Patent Application No. 2003-269393